Interface Composition

Demonstrating Interface Composition in Go


March 9, 2023

One cool feature of Go is that it lets you compose interfaces using other interfaces. The prototypical example of this is the ReadWriter, which is composed of the Reader interface and the Writer interface.

Below is an example of a toy program that defines a UserStorer interface and an ItemStorer interface, then a Storer interface that is composed of these interfaces

Define User Stuff

package main

import (

//create type
type User struct {
    Name string
    Age  int

//create interface
type UserStorer interface {
    AddUser(users []*User, user *User) []*User

//define an empty struct that will implement interface
type ustore struct{}

func NewUserStore() UserStorer {
    return &ustore{}

func (us ustore) AddUser(users []*User, user *User) []*User {

    users = append(users, user)

    return users

Define Item Stuff

//define type
type Item struct {
    Name  string
    Usage string

//define interface
type ItemStorer interface {
    AddItem(items []*Item, item *Item) []*Item

//define empty struct that will implement interface
type istore struct{}

//define constructor
func NewItemStore() ItemStorer {
    return &istore{}

//define add method
func (is istore) AddItem(items []*Item, item *Item) []*Item {

    items = append(items, item)

    return items

Define Storer Stuff

type Storer interface {

type myStore struct {

func NewStorer(us UserStorer, is ItemStorer) Storer {

    s := &myStore{
        UserStorer: us,
        ItemStorer: is,

    return s

Create Simple Program

func main() {

    us := NewUserStore()
    is := NewItemStore()

    s := NewStorer(us, is)

    var items []*Item
    var users []*User

    u := &User{
        Name: "Eric",
        Age:  34,

    i := &Item{
        Name:  "Phone",
        Usage: "texting",

    items = s.AddItem(items, i)
    users = s.AddUser(users, u)

    //embed to json
    ji, _ := json.Marshal(items)
    ju, _ := json.Marshal(users)

    //print out results